Even though current technologies of fiber laser have made significant progress toward achieving a compact and reliable fiber laser system providing high quality output laser with ever increasing output energy, however those of ordinary skill in the art are still confronted with technical limitations and difficulties. Specifically, in a fiber laser system implemented with the Chirped Pulse Amplification (CPA) for short pulse high power laser amplifier, the non-linearity comes up naturally with the short pulse fiber laser. Due to the fact that the non-linearity effects are inherent with the short pulse laser fiber, and the removal of such non-linearity appears to be impossible, the bottom line problem is not to try to eliminate the non-linearity. Rather, the major thrust of research efforts now is directed toward a solution in finding a method to manage the non-linearity.
For a fiber laser system, the phenomenon of laser dispersion in a fiber laser system is confronted with the same difficulties with the same situation. The dispersion is inherently generated during the transmission and amplification of the laser in the fiber laser system. Furthermore, it is also practically impossible to remove the dispersion from a fiber laser system. Therefore, the key and the central idea is not to eliminate the nonlinearity and dispersions but to obtain the best pulse shape as that is necessary to control the nonlinearity and dispersion in the whole fiber amplifier system. However, the conventional technologies for configuring a high energy, short pulse fiber laser system have not yet provided a solution to effectively manage and resolve such difficulties.
FIG. 1 is a functional block diagram for illustrating a conventional short pulse fiber laser amplifier, namely, the fiber chirped pulse amplification (CPA) system. The fiber chirped pulse amplification laser system includes a mode-locking fiber laser 10, a fiber stretcher 15 to stretch the laser pulse to generate a long pulse, a preamplifier 20 to get high average power, pulse picker 25 to decrease the repetition rate so that real high energy is possible, an amplifier chain 30 to obtain high energy and a compressor 35 to generate pulses with short pulse duration.
In a high-energy short-pulse fiber laser system, the generation of dispersion is more than a passive effect. In the fiber laser system, besides the polarization mode dispersion (PMD), the material and waveguide dispersion are two main passive sources, which will affect the pulse shaping dynamics. The passive dispersion can be classified according to the mathematical expression and relevant importance for the pulse reshaping dynamics, as the group velocity dispersion (GVD) and the third order dispersion (TOD). The GVD and the TOD are introduced by the fiber components. A conventional single mode fiber stretcher may accumulate very high passive dispersion, including GVD and TOD and the pulse duration is then largely stretched accordingly. The stretched pulse is partly compressible without further amplification. FIGS. 2A and 2B illustrate the effect of passive dispersion on the pulse reshaping wherein FIG. 2A shows the pulse has a large pedestal due to the passive TOD dispersion effects. Although as shown in FIG. 2A, the center peak is still narrow, but the pedestal is getting quite high. In FIG. 2B, the corresponding spectrum as shown can actually support much shorter pulse with the time-bandwidth product as high as 3.7 and yet it is difficult to realize.
On the other hand, the nonlinearity not only generates more spectra via SPM and Stimulated Raman Scattering (SRS) thus inducing self-focusing under extreme peak power, the nonlinearity can also introduce drastically large chirp into the pulse. The large chirp might change the pulse shaping process permanently that is generally referred to as the nonlinearity chirp. Comparing with the passive dispersions, the nonlinearity chirp has different evolution dynamics and functionality for the pulse reshaping process. The pulse reshaping can be constructive or destructive and the pulse can be re-compressible, or it could be unable to get re-compressed. Conventional laser fiber systems still have limitations to assure that the laser transmitted in the system can be continuously maintained as a re-compressible pulse.
For a CPA fiber system shown in FIG. 1, two previously filed Patent Applications 60/062,905, and 60/082,705 filed by one the Applicants of this invention, a disclosure was made to use the nonlinear effect, i.e., the Self-Phase-Modulation (SPM), in the stretcher stage and after amplifier stage to improve the compressibility and to achieve shorter and shorter pulse, and apply these ideas in our laser system. However, there are still limitations in improvements that can be achieved through such fiber systems.
Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide fiber laser to manage the nonlinearity and the dispersion generated in the laser system such that the above-discussed difficulty may be resolved. Specifically, since the nonlinearity and the dispersion phenomena are encountered in the entire fiber system, it is further desirable to provide new and improved system configurations and method to manage the nonlinearity and the dispersion over the entire fiber system and over both high and low levels of laser energies to achieve even further improvements for even boarder scopes of applications.